1. Field of the Invention
The present invention relates to an acoustic wave device used as, for example, a band-pass filter. More particularly, the present invention relates to a resonator-type acoustic wave device in which first and second acoustic wave filters are cascade connected with each other.
2. Description of the Related Art
Band-pass filters used at the transmission side of mobile phones must have greater amounts of attenuation in reception-side passbands. In contrast, for example, personal communication systems (PCSs) must have sufficiently small insertion losses in the passbands and sufficiently large amounts of attenuation in blocking bands near the passbands because of the narrow interval between the transmission bands and the reception bands. Accordingly, such systems must have sufficiently steep filter characteristics.
Japanese Unexamined Patent Application Publication No. 2000-349590 discloses a surface acoustic wave device capable of increasing the amount of attenuation in the blocking band near the high-frequency side of the passband. FIG. 17 is a schematic plan view of the surface acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 2000-349590.
A surface acoustic wave device 1001 includes a piezoelectric substrate 1002. An electrode structure shown in FIG. 17 is provided on the piezoelectric substrate 1002 to define the surface acoustic wave device in which first and second longitudinally-coupled resonator-type surface acoustic wave filters 1003 and 1004 are cascade connected with each other.
Specifically, the first surface acoustic wave filter 1003 is cascade connected with the second surface acoustic wave filter 1004.
The first surface acoustic wave filter 1003 includes a first interdigital transducer (IDT) 1003a and second and third IDTs 1003b and 1003c that are arranged on either side of the first IDT 1003a in the direction in which surface waves propagate. Reflectors 1003d and 1003e are arranged on either side of the portion in which the IDTs 1003a to 1003c are arranged in the direction in which the surface waves propagate.
Similarly, the second surface acoustic wave filter 1004 includes a first IDT 1004a, second and third IDTs 1004b and 1004c that are arranged on either side of the first IDT 1004a in the direction in which the surface waves propagate, and reflectors 1004d and 1004e. Referring to FIG. 17, one end of the IDT 1003a in the first surface acoustic wave filter 1003 is connected to an input electrode pad 1005 and the other end thereof is electrically connected to a ground electrode pad 1006. One end of the second IDT 1003b is connected to one end of the third IDT 1003c via a connection electrode 1007. The other end of the second IDT 1003b is connected to a first signal line 1008 and the other end of the third IDT 1003c is connected to a second signal line 1009. A surface acoustic wave resonator 1010 is provided between the first surface acoustic wave filter 1003 and the second surface acoustic wave filter 1004. The surface acoustic wave resonator 1010 includes an IDT and first and second reflectors arranged at either side of the IDT.
The first signal line 1008 and the second signal line 1009 are connected to one end of the IDT of the surface acoustic wave resonator 1010. Third and fourth signal lines 1011 and 1012 are connected to the other end of the IDT of the surface acoustic wave resonator 1010. The third signal line 1011 is connected to one end of the second IDT 1004b in the second surface acoustic wave filter 1004. The fourth signal line 1012 is connected to one end of the third IDT 1004c. The other end of the second IDT 1004b is connected to the other end of the third IDT 1004c via a connection electrode 1013. One end of the central first IDT 1004a is connected to a ground electrode pad 1014 and the other end thereof is connected to an output electrode pad 1015.
In the surface acoustic wave device 1001, the impedance characteristics at an anti-resonance point of the surface acoustic wave resonator 1010 can be used to increase the amount of attenuation at the high-frequency side of the passband and to increase the steepness of the filter characteristics at the high-frequency side of the passband.
However, since it is necessary to connect the surface acoustic wave resonator 1010 in series between the first and second surface acoustic wave filters 1003 and 1004 in the surface acoustic wave device 1001 described in Japanese Unexamined Patent Application Publication No. 2000-349590, it is necessary to provide a sufficiently large space between the first and second surface acoustic wave filters 1003 and 1004. In other words, since it is necessary to arrange the surface acoustic wave resonator 1010 between the first and second surface acoustic wave filters 1003 and 1004, to connect the first and second surface acoustic wave filters 1003 and 1004 to the surface acoustic wave resonator 1010 via the signal lines 1008, 1009, 1011, 1012, etc., and to provide the ground electrode pads 1006 and 1014, it is necessary to provide a large amount of space between the stages. Accordingly, it is difficult to reduce the size of the surface acoustic wave device 1001.
In order to resolve such a problem, the surface acoustic wave resonator may be connected in series between either of an input terminal and an output terminal and the first surface acoustic wave filter or the second surface acoustic wave filter in a structure in which first and second longitudinally-coupled resonator-type surface acoustic wave filters are cascade connected with each other. However, the symmetry between the configuration from the input terminal to the first surface acoustic wave filter and the configuration from the second surface acoustic wave filter to the output terminal is adversely affected in such a structure. In other words, since the configuration at the input side is asymmetric with the configuration at the output side, it is necessary to design the surface acoustic wave filter at each stage in order to achieve the impedance matching. However, a specific design method for the first and second surface acoustic wave filters is unknown in the above configuration.
In recent years, various boundary acoustic wave devices utilizing boundary acoustic waves have been proposed, instead of the surface acoustic wave devices. Also in the boundary acoustic wave devices, a specific design method for first and second surface acoustic wave filters to improve the filter characteristics by using a similar structure is unknown.